Whether fertility in the less developed countries continues at current levels or declines drastically, the expected numbers in the 15 to 65 age group—the main labor-force ages—would be practically unchanged for 15 years and the rate of increase would vary only by about 5 percentage points over 20 years. For the main school-going ages of 5 to 15, a 5 percentage point effect after 10 years compares with one of well over 25 points in 20 years, with very similar orders of effect for total population, hence consumers, over these same time spans.
Persuasive explanatory or predictive models for dealing with demographic-economic interactions during or beyond such prolonged time intervals have not been devised.
In developed areas, fluctuations in fertility, rather than sustained levels or trends, have been predominant determinants of population growth rates during recent decades. The main impacts of such fluctuations often have been upon short-term aggregate demand rather than upon long-term supply. Here too, however, nondemographic factors—for example, employment policy, investment propensities, and foreign trade—have been important enough to command greater attention from policymakers and analysts. As in the less developed countries, the macro-economic effects of demographic trends typically require longer time intervals to acquire sufficient causal significance than do many of the main nondemographic variables.
These comments help explain why attention to "population problems" has been so largely of a "crisis" variety, in both the developed and less developed regions. A further and more serious possibility is that adverse effects of rapid population growth on economic factors are likely to be understated or discounted, as compared to nondemographic factors. Whether or not such impacts are fully discoverable, even in principle, their presumptive longer-run magnitudes and probabilities are nevertheless sufficiently established to accord them a high priority, once the relevant, and distinctive, time spans surrounding their operation are given their due weight.
Research over the past 2 decades on the relations between long-term trends in production and traditional factor inputs (land, labor, and capital) suggests that these factor inputs have accounted for surprisingly small parts of the observed growth in output. After allowances are made for changing amounts of labor and capital (including land), well over half of such increases in output in a number of countries have been identified as "residual," i.e., statistically unexplained. The inference is that other factors, in particular improvements in the quality of the labor force and general technological change, have been the foremost determinants of economic growth in modern industrial history.
This line of reasoning could have important implications for understanding demographic-economic interrelations. It would relegate to a much more secondary position the role of diminishine returns: it would notelated collective consumption needs would stay largely unchanged for the better part of a decade, and